Radio transmitter



R. c. SHAW 2,085,125

RADIO TRANSMITTER Filed June 26, 1935 2 Sheets-Sheet 1 June 29, 1937.

FIG.

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. E 4 R MODULATOR AMPLIFIER II AMPLIFIER 05C R. E 4 AMPLIFIER "000M TORAMPLIFIER 8 M- //Vl E/VTOR R. C. SHAW V ATTORNEY June 29, 1937. SHAW2,085,125

RADIO TRANSMITTER Filed June 26, 1935 2 Sheets-Sheet 2 50 AF. Fla 3IIFAMPL/F/ER H l- H/Gh' PJ uoouuron AMPLIFIER POWER 55 A AMPLIF/ER 05c.

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-H- Bf? \1; 45 43 lNVEA/TOR R. C. .5 HA W A 7' TORNEJ Patented. June 29,1937 omrso s'mrss PATENT OFFICE RADIO TRANSMITTER Application June 26,

3 Claims.

This invention relates toradio transmitters and particularly tofeed-back circuits for controlling the power radiated by radiotransmitters.

One object of the invention is to provide a radio transmitter with afeed-back circuit that shall control the transmitter to maintain theradiated power substantially constant.

Another object of the invention is to provide a radio transmitter with afeed-back circuit that shall control the transmitter to maintain theradiated power substantially constant and with a feed-back circuit forstabilizing the audio frequency stages to improve distortion and noiseconditions.

Another object of the invention is to provide a radio transmitter with afeed-back circuit that shall control the transmitter according to thedirect current component of the carrier envelope to maintain theradiated power substantially constant.

A further object of the invention is to provide a radio transmitterhaving radio frequency stages comprising space discharge devices with afeedback circuit from a pick-up antenna that shall control the grid of adevice in a radio frequency stage according to the direct currentcomponent of the carrier envelope to maintain the radiated powersubstantially constant.

In radio telephone transmitters it is desirable to hold the radiatedpower output at a constant level. Broadcasting stations and ultra shortwave systems would be improved in operation and in the quality of theprograms radiated if the carrier amplitude variations arising fromdirect current power changes or from other causes were eliminated. Ifthe radiated power was maintained constant, systems for guidingaircraft, such as blind landing devices and altimeters, which dependupon the absolute value of a transmitted carrier for accuracy, would bemade more reliable. The need for constant carrier amplitude in the caseof altimeters cannot be over-emphasized. In addition to the abovecommercial circuits where constant carrier amplitude is desirable, theradiation of a constant amplitude carrier is desirable in making fieldstrength observations, in making studies of fading and in making waveangle tests.

In short wave radio circuits, it is normally desired to hold thetransmitter modulation constant for a fixed amplitude of speech levelfrom the speech input equipment. If the radio frequency power varies,the per cent modulation will also vary and usually produce undesirableeffects,

such as distortion. This distortion will be re- 1935, Serial No. 28,421

duced and the modulation percentage will be maintained more constant ifthe radio frequency power is held constant.

In ultra short wave telephone systems, the radio link should be entirelyautomatic in operation as in the case of telephone repeaters. In thistype of circuit, the ratio of the received power to the transmittedpower is constant and the power radiated by the transmitter should alsobe constant. If the power output from the transmitter is maintainedconstant regardless of direct current power changes and temperaturechanges, the resultant effect will be a radio circult of constant gain.

In accordance with the present invention, a negative feed-back circuitis provided for controlling a radio transmitter to maintain the radiatedpower substantially constant. In controlling the power output, it isproposed to feed back only the direct current component of the carrierenvelope. In one form of the invention, a pick-up antenna is positionedin the vicinity and preferably in the line of fire of a transmittingantenna connected to a radio transmitter having the usual audiofrequency stages and radio frequency stages. One of the radio frequencystages comprises a space discharge device hav ing a control grid, ananode, a cathode, and a screen grid. The potential on the screen grid iscontrolled in accordance with the direct current component of thecarrier envelope picked up by the pick-up antenna. A feed-back circuitis connected between the pick-up antenna and the screen grid of thespace discharge device in the radio frequency stage. In one form of theinvention, the feed-back circuit comprises a rectifier of any suitabletype which controls the potential impressed on the grid of athree-element space discharge device. A fixed negative bias is impressedupon the grid of the three-element space discharge device and a variablepositive potential is impressed on the grid according to the strength ofthe carrier wave received by the pick-up antenna. The impedance of thethreeelement space discharge device forms a part of a potentiometerwhich is connected to the screen grid of the space discharge device inthe radiofrequency stage of the transmitter. A filter comprisingcondensers associated with resistance is included in the feed-backcircuit for insuring that speech or syllable frequencies will notcontrol the screen grid of the radio frequency speech discharge device.The time constant of the filter should be of the order of one to tenseconds.

If the amplitude of the carrier envelope radiated by the transmittingantenna is above normal value, the rectified potential from the pickupantenna which is impressed upon the grid of the three-element spacedischarge device will be raised to lower the impedance of thethree-element space discharge device and decrease the potentialimpressed on the screen grid of the radio frequency space dischargedevice. A decrease in the potential on the screen grid will result inlowering the power output from the transmitting antenna.

In another form of the invention, the pickup antenna which is located inthe vicinity of the transmitting antenna may be connected directly to athree-element space discharge device Without the interposition of arectifier. The three-element space discharge device performs thefunction not only of detection but also of control for the potentialimpressed on the screen grid of the radio frequency device.

In another form of the invention, a pick-up antenna located in the lineof fire of the transmitting antenna and preferably in the vicinity ofthe transmitting antenna is connected by a feed-back circuit to thecontrol grids of one or more radio frequency devices in a radiotransmitter. The feed-back circuit comprises a rectifier of any suitabletype connected to the pickup antenna which impresses potential in a reverse phaseupon the control grids of one or more of the radio frequencydevices in the transmitter. The radio frequency devices are of thevariable mu type. The feed-back circuit, as in the case of the othermodifications, is provided with a filter to insure control by thefeed-back circuit only in accordance with the direct ourrentcomponent ofthe carrier envelope.

In still another form of my invention, the pickup antenna may have twofeed-back circuits connected to it. One of the feed-back circuits servesto control the power output radiated from a transmitting antenna and theother feed-back circuit serves to control the audio frequency stages ofa transmitter to improve distortion and noise conditions. Filters areprovided in the feed-back circuits to insure control by one feedbackcircuit only in accordance with the direct current component of thecarrier envelope and control of the other feed-back circuit only inaccordance with the audio frequency components. Feed-back circuits forcontrolling the distortion and noise are disclosed in the application ofE. B. Ferrell, Serial No. 729,735, filed June 9, 1934.

In the accompanying drawings,

Fig. 1 is a diagrammatic view of a radio transmitter provided with afeed-back circuit constructed in accordance with the invention;

Fig. 2 is a diagrammatic view of a transmitter provided with amodification of the feed-back circuit shown in Fig. 1; i

Fig. 3 is a diagrammatic view of a radio transmitter provided with anegative feed-back circuit for governing the control grid of a radiofrequency amplifier stage in accordance with the invention; and

. Fig. 4 is a diagrammatic view of a radio transmitter provided with afeed-back circuit for controlling the radiated power and a feed-backcircuit for controlling an audio frequency stage to prevent distortionand noise.

Referring to Fig. l of the drawings, a radio transmitter is shownconnected to a transmitting antenna 2 by means of a coupling transformer3, The transmitter comprises an oscillator l of any suitable type, aradio frequency amplifier 5, a modulator 6, and a radio frequencyamplifier space discharge device I which is shown in detail. Atransmitter 8 is connected by suitable audio frequency amplifiers 9 tothe modulator 6. The amplifier comprises a control grid H), a screengrid l I, an anode l2, and a cathode !3. The potential on the screengrid II of the device 1 is controlled by a negative feedback circuit Mwhich is connected to a pick-up antenna l5. The pick-up antenna i5 islocated in the line of fire of the transmitting antenna 2 and preferablyis located in the vicinity of the transmitting antenna 2. The feed-backcircuit I4 comprises a rectifier l6 connected around a choke coil H inthe circuit of thepick-up antenna l5. The rectifier It may be of anysuitable type and preferably is a two-element space discharge device.The rectified current from the rectifier l6 controls the potentialimpressed on the grid of a three-element space discharge device I8, theimpedance of which forms part of a potentiometer 59 for governing thepotential impressed on thescreen grid H of the device I.

The three-element space discharge device l8 comprises a control grid 20,an anode 2!, and a cathode 22. A battery 23 is provided for impressing anegative bias on the grid 20. A potentiometer 24 included in the circuitof the rectifier I 6 impresses a variable positive potential on the grid2e for controlling the impedance of the device it. Two condensers 25 and26 cooperate with the potentiometer 24 and a resistance 2? to have afiltering action and prevent control of the device I8 by speech orsyllable frequencies. The time constant of the filter is of the order ofone to ten seconds. This filter insures control of the device is inaccordance with the direct current component of the carrier envelope.

One section of the potentiometer l9 comprises a resistance element 28and the other section thereof comprises the impedance of the spacedischarge device iii. A battery 29 is connected to the potentiometer anda choke coil 30 is included in circuit therewith. A condenser 35 servesas a by-pass condenser. Negative bias for the grid it of the device 7 issupplied by a battery 32 and plate potential for the anode I2 issupplied by battery 33.

If the power output from the transmitting antenna 2 is raised abovenormal value, the voltage in the pick-up antenna [5 increases. Thisincrease in voltage in the pick-up antenna increases The rectifiedpotential opposes the bias of the battery,

the potential rectified by the rectifier I5.

back circuit M will be reversed to raise the power' output fromthetransmitting antenna. A decrease of the power output below normalwill decrease the potential rectified by the rectifier l5 which willincrease the negative potential on the grid 20 of the device i8 whichwill increase the impedance of the device l8. Increasing the impedanceof the device IE will increase the potencreases below normal value.

. as and 45.

. Potential is fed back tial on the screen grid H and increase the radiofrequency power radiated from the transmitting antenna 2.

In Fig. 2 of the drawings is shown a modification of the circuit shownin Fig. 1. The radio transmitter in Fig. 2 is exactly the same as theradio transmitter shown in Fig. l and similar parts have been indicatedby like reference characters. The feed-back circuit 34 shown in Fig. 2between the pick-up antenna l5 and the screen grid l l. of the spacedischarge device 1 comprises a three-element space discharge device 35which has not only the function of detection but also that of control.The device 35 comprises control grid 36, an anode 31, and a cathode 38.The pickup antenna is directly connected to the control grid 36 and thescreen grid H of the device I is connected through the choke coil 39 tothe cathode 38 of the device 35.

The voltage impressed on the screen grid I I of the device I is variedaccording to the impedance of the control device 35. The output from thespace discharge amplifier l varies inversely according to the value ofthe impedance of the device 35. The impedance of the device 35 isdependent on the potential impressed on the grid 35. The choke coil [1prevents the radio frequency voltage from being by-passed to ground bycondenser 39. The grid bias for the device 35 is obtained from the radiofrequency carrier which is rectified by grid leak detection in thedevice 35. Grid bias for normal operation is obtained by varying theamount of radio frequency voltage on the grid 36 of the device 35. Ifthe power output radiated from the transmitting antenna 2 is increasedabove normal value, the rectified direct current voltage of the device35 will cause the grid to go more negative and the plate-to-filamentresistance thereof will increase. The potential on the screen grid ll ofthe device I will decrease, causing the carrier output from thetransmitting antenna 2 to decrease to normal. Theabove effect isreversed if the amplitude of the carrier de- Co-ndensers 39 and 49 inthe feed-back circuit 34 serve to insure control by the feed-backcircuit only in accordance with the direct current component of thecarrier envelope.

Referring to Fig. 3 of the drawings, a feed-back circuit from a pick-upantenna 41 is shown for governing the control grids of variable mu tubesin a radio frequency transmitter 42. The transmitter 42 comprises anoscillator 53 connected to a three-element space discharge device 55 ofthe variable mu type, a space dischargedevice 45 similar to the deviceM, a modulator 46, and radio frequency amplifiers G1 and 48 connected tothe transmitting antenna 49. Audio frequency amplifier stages 59 serveto connect a transmitter 5| to the modulator 46. A battery 52 isprovided for impressing a negative bias on the grids 53 and 54 of thedevices M and 55. A rectifier 55 is connected across a choke coil 56 anda potentiometer 5'! in the circuit of the pick-up antenna 4|. from thepotentiometer 51 for controlling the grids 53 and 54 of the device Acondenser 58 is provided for bypassing low frequency currents and togive the feed-back circuit a time control so that the control of thegrids 53 and 54 is effected solely in accordance with the direct currentcomponent of the carrier envelope. The circuit shown in Fig. 3 of thedrawings operates in the same manner as the circuit shown in Fig. 1except that the phase control is reversed. With an increase in theradiated power from the transmitting antenna 49, the rectified currentsupplied by the rectifier 55 increases for supplying negative potentialwhich is impressed on the grids 53 and 54. The negative potentialimpressed on the grids 53 and 54 lowers the power radiated by thetransmitting antenna 49. A reverse operation takes place if the powerradiated by the transmitting antenna 49 is lowered below normal value.

Referring to Fig. 4 of the drawings, a radiotransmitter similar to theradio transmitter shown in Fig. 3 is illustrated provided, not only witha feed-back circuit for controlling the radiated power as in the circuitshown in Fig. 3,but also with an auxiliary feed-back circuit forcontrolling the audio frequency stages to correct for noise anddistortion. Like parts shown in Fig. 4 to those shown in Fig. 3 Will beindicated by similar reference characters. The audio frequency amplifiershown in Fig. 4 comprises two space discharge devices 59 and 69 whichare, for example, of the three-element space discharge type. The grid 6iof space discharge device 59 is controlled by a feed-back circuitconnected by condenser 62 to the potentiometer 51 in circuit with therectifier 55. The condenser 62, in combination with the choke coil 56 inthe potentiometer 51, insures control of the grid 6| of the device 59solely in accordance with the audio frequency currents. The condenser 63connected to the feed-back circuit for controlling the radio frequencydevices 44 and 45 serves as a filter to insure against control of thedevices 44 and 45 by audio frequency currents or syllable frequencycurrents. Feedback circuits for controlling audio frequency devices tocorrect for distortion and noise are disclosed in the above-mentionedapplication of E. B. Ferrell, Serial No. 729,735 filed January 9, 1934.

Modifications of the circuits and the arrangement and location of partsmay be made within the spirit and scope of the invention. Suchmodifications are intended to be covered by the appended claims.

What is claimed is:

l. In a radio circuit, a transmitting antenna, a radio transmittercomprising radio frequency stages connected to said antenna and audiofrequency stages connected to said radio frequency stages, a pick-upantenna located in the direction of propagation of said transmittingantenna, means comprising a feed-back circuit from said pick-up antennato a radio frequency stage for controlling said transmitter to maintainthe carrier wave amplitude substantially constant, and means comprisinga degenerative feed-back for feeding back a wave opposite in phase tothe signal wave from said pick-up antenna to an audio frequency stage tocontrol the audio stage by audio frequency currents and reduce noise anddistortion of the envelope of the Waves radiated from the transmittingantenna.

2. In a radio circuit, a transmitting antenna, a radio transmittercomprising 'radio frequency stages and audio frequency stages connectedto said antenna, a pick-up antenna located in the vicinity of saidtransmitting antenna, a rectifier connected to said pick-up antenna,means comprising a feed-back circuit from said rectifier to a radiofrequency stage for feeding back only the direct current component ofthe carrier wave to control said radio stage and maintain the carrierwave amplitude substantially constant, and means comprising adegenerative feed-back circuit from the rectifier to an audio frequencystage for controlling said audio frequency stage by audio frequencycurrents to reduce envelope disone of said radio: frequncvamplifierdevices for tortion of the radiated wave. controlling said device tomaintain thecarrier 3. In a radio circuit, a transmitting antenna, awave amplitude substantially constant, and radio transmitter comprisingradio frequency means comprising a gain reducing feed-back quencycurrents, a pick-up antenna located in the and reduce envelopedistortion of the modulated vicinity of said transmitting antenna, arectifier Wave. 10 connected to said pick-up antenna, means com- ROBERTC. SHAW.

